In a known General Packet Radio Service (GPRS) mobile communications network, a Gateway GPRS Support Node (GGSN) acts as a gateway to and from an Internet Service Provider (ISP), and has a connection to a general data communications network. Connected to the GGSN is a Serving GPRS Support Node (SGSN), which is further connected to multiple Base Station Systems (BSS).
A mobile station (MS), which is active within the network, has a connection to one BSS, and the SGSN handles the routing of data from the GGSN to the BSS.
Each BSS has at least one Packet Control Unit (PCU), which handles the different GPRS users, and schedules data on the radio resources which are available for GPRS users in the cell.
When downlink data is transmitted to an MS in a GPRS network, the data is buffered, in the form of Logical Link Control (LLC) Packet Data Units (PDU) both in the SGSN and in the BSS. The LLC is the protocol which provides a logical link between the MS and the SGSN.
When a Mobile Station moves in the area covered by the network, procedures are provided to control the way in which its connection changes from one cell to another.
The BSS specifies a Network Control (NC) mode, and broadcasts this to the Mobile Station on control channels. In Network Control mode 0 (NC0) or Network Control mode 1 (NC1), the MS may perform cell reselection autonomously, as described in the 3rd Generation Partnership Project Technical Specification 3GPP TS 44.060 V5.0.0, section 5.5.1.1. Thus, the MS measures the signal strengths of neighbouring cells and, when it determines that a cell change is appropriate, it performs the cell change. The BSS determines that the MS has changed cell only when it receives a Cell Update message from the MS in the new cell.
In Network Control mode 2 (NC2), the cell reselection is initiated by the network, as described in the 3rd Generation Partnership Project Technical Specification 3GPP TS 44.060 V5.0.0, section 8.4. Thus, the MS measures the signal strengths of the serving cell and of neighbouring cells, and sends measurement reports to the BSS. When the BSS determines that a cell change is appropriate, it directs the MS accordingly.
A cell change, as described above, whether performed autonomously by the MS or initiated by the BSS, typically takes 3-5 seconds, during which time data transfer is interrupted.
If it is supported by the MS, the functionality Network Assisted Cell Change, as described in the 3rd Generation Partnership Project Technical Specification 3GPP TS 44.060 V5.0.0, sections 5.51.1a and 8.8, can reduce this interruption to less than 1 second. In this case, when the MS determines that a cell change is appropriate, it sends a Packet Cell Chance Notification message to the BSS. The BSS responds with the system information for the proposed cell reselection, thereby allowing the MS to make a faster access in the new cell.
However, there remains an interruption, during which there is no data transfer. Further, the interruption may lead to timeouts and/or reduced data throughput in higher layer protocols.
Moreover, when an SGSN detects a cell change, it sends a FLUSH-LL PDU message to the old BSS, as described in the 3rd Generation Partnership Project Technical Specification 3GPP TS 08.18 V8.9.0, section 8.1. In some cases, this will allow buffered data, awaiting transmission to the MS, to be transferred to the PCU for the new cell. However, in other cases, particularly in the case of a cell change between routing areas or between Network Service Entities, the buffered data cannot be transferred, and must be retransmitted at a higher layer protocol. Again, this can lead to reduced data throughput for the MS.